The present invention relates to a method and an apparatus for measuring the amount of ink discharged, a printing apparatus-and a method of measuring the amount of ink discharged by the printing apparatus, and more particularly, to a method and an apparatus for measuring the amount of ink discharged by single discharging operation of a printhead, such as that used in an ink-jet printer, having fine nozzles which discharge a very small amount of ink from each nozzle.
For an apparatus such as the ink-jet printer which has a printhead consisting of a plurality of fine nozzles, it is quite important to control the amount of ink discharged from each nozzle to be uniform in order to stabilize printing quality. For this purpose, it is desired to correctly and instantly obtain the amount of ink discharged from each nozzle.
There are known methods of measuring a small amount of colored ink droplet (e.g., color ink), namely, (1) a weighing method and (2) an absorbance method.
Followings are explanation when the amount of ink discharged from a nozzle of an ink-jet printer is measured in the above two methods.
(1) Weighing Method
In this method, ink is discharged at a fixed interval for a predetermined time period, and the amount (weight) of used ink in the time period is measured by a chemical balance or the like (care must be taken to minimize vaporization vaporization of ink during this operation). Thereafter, by dividing the amount of ink used by the number of discharging operation performed, an average amount of ink droplet (an average discharging amount) in each discharging operation can be obtained.
(2) Absorbance Method
This method uses the relationship between concentration of solution and light absorption, which is known as Lambert-Beer's law. More specifically, solution having a certain concentration is poured into a transparent container having a fixed depth, and light having intensity I.sub.0 is incidented on the container from one side, then intensity I of the light transmitted through the solution in the container is measured. Since a part of the incidented light is absorbed in the solution in the container, the intensity of the light reduces while passing the solution. It is known that the intensity is lowered in proportion to the concentration of the solution. Defining A as absorbance, the relationship of this law can be expressed in the following equation: EQU A=-Log(I.sub.0 /I)=a.multidot.b.multidot.c
where a is a slope, b is the depth of the solution and c is the concentration of the solution. By using this equation, a calibration line showing the relationship between concentration and absorbance for the ink to be used is obtained in advance Next, ink is discharged from a nozzle toward a transparent solution of a known volume (a solution which has a light absorbance as small as possible is preferred), then absorbances by the solution containing discharged ink after each discharging operation, namely absorbances corresponding to the number of discharging operations, are measured. From the measured absorbances and the calibration line which was obtained in advance, the concentration of the solution including ink is determined. Then, the amount of ink mixed in the solution can be found by taking the amount of the original solution into consideration. The obtained amount of ink is divided by the number of discharging operations, thereby finding an average amount of ink discharged by each discharging operation.
However, according to the foregoing two conventional methods, there are problems in which a certain amount of ink has to be discharged; thus, it is impossible to instantly obtain the amount of ink discharged from each nozzle. As an experiment, the applicant of the present invention measured the amount of ink discharged per nozzle by the weighing method, and found that ink corresponding to 500,000 dots were discharged, and it took 12 minutes for the measurement. Thus, it takes quite a long time to measure the amount of ink discharged per nozzle of an ink-jet printer having an ink-jet printhead consisting of 64 or 128 fine nozzles. Furthermore, in the foregoing two methods, since the amount of ink per discharging operation is averaged, it is impossible to measure the real amount of ink discharged per nozzle in a single discharging operation.